Governor K Rosaiah administered him the oath of office and secrecy at the Raj Bhavan here.
Chief Minister Jayalalithaa and her cabinet colleagues were present on the occasion.
Venkatachalam, representing Perundurai constituency, was appointed Minister last night after his predecessor and senior leader K A Senkottaiyan, considered "No.3" in the party ranks, was dropped from the Ministry by Jayalalithaa.
He was also sacked as AIADMK Headquarters Secretary, triggering speculation in political circles over Jayalalithaa giving the boot to such a senior associate.
